The overall objective of this project is to evaluate irradiation as a modality for immunosuppression for bone marrow and organ transplantation. Emphasis will be on comparison of total body irradiation (TBI) and total lymphoid irradiation (TLI) as immunosuppressive agents. TBI and TLI will be used alone and in combination with immunosuppressive drugs, e.g., cyclophosphamide. Studies will be done in the mouse and will compare single dose with fractionated dose irradiation. The relative biologic effectiveness of single dose and fractionated dose TBI and TLI will be compared. Organ grafts will include skin and bone marrow from syngeneic and allogeneic donors. Graft rejection will be studied in vivo and in vitro. Graft versus host reactions (GVHR) will be studied in detail to evaluate the hypothesis that TLI will be associated with less GVHR morbidity and mortality than TBI.